#include<unistd.h>
#include<stdio.h>
#include<stdlib.h>
#include<signal.h>
void sig_alarm() {
    printf("over.\r\n");
    exit(0);
}
int main() {
    signal(SIGALRM,sig_alarm);
    alarm(5);
    sleep(10);
    printf("hello world.\r\n");
    return 0;
}
